In high-speed data communication, for example multimedia communication, data transmitters and receivers are required to work at high frequencies, for example in the Giga Hertz (GHz) RF range. In such data transmitters and receivers, RF power amplifiers are an important element to create sufficient signal power with high linearity at RF frequencies.
Bipolar transistors such as Heterojunction bipolar transistor HBTs are preferred to CMOS based devices for power amplifier applications due to their superior performance, such as higher output signal power, high power density and high linearity, at RF frequencies. On the one hand, III-V based transistors are preferred over Si based devices due to their higher breakdown voltage, higher maximum operating frequency and higher power density. On the other hand however Si based HBT devices, such as SiGe are more cost effective and are more easily integrated into for example larger scale system on chip than the III-V based devices, albeit at the cost of lower breakdown voltages and lower output powers, when compared to such III-V based devices.
In traditional SiGe based HBT devices attempts have been made to increase the breakdown voltage by implementing collector implants. However increasing the breakdown voltage in this way requires a trade-off of lowering the cut-off frequency which is the high-frequency figure of merit of a HBT device. This trade-off can be explained by the so-called Johnson limit. The Johnson limit equates to the product of peak current gain cut-off frequency fT and the collector-emitter breakdown voltage BVCEO. These characteristics are typically controlled by controlling the amount of doping in the collector of a particular HBT device. A high collector doping level on the one hand increases fT because it postpones the so-called Kirk effect, but on the other hand it decreases BVCEO because it increases the local electric field at the collector.
The trade-off between breakdown voltage and cut-off frequency may limit the application of such HBT devices. For example such devices may be unsuitable for RF amplifiers which are required to operate at medium to high power and high linearity at sufficiently high frequencies in the GHz range.
Attempts to improve device performance by reducing the effects of the above mentioned trade-off include for example devices comprising so-called gated bottom up-collectors or gated lateral collectors. Such devices attempt to reshape the electric field distribution to effectively increase the breakdown voltage without lowering the cut-off frequency fT.
For devices comprising gated bottom up-collectors, complexities in the device processing, for example the processing of shallow trench isolation regions with different depths, may result in increased processing time and increased device cost.
For gated lateral collectors which do not comprise a shallow trench isolation (STI) region, a very high localised electric field, in the region of 600 KVcm−1 to 800 KVcm−1, may occur at the junction of the base and the collector. This electric field, commonly known as a field spike, may result in device reliability issues for example, device breakdown due to impact ionization. Whenever the field is higher than critical field for the device material (for example 500 kV/cm in Silicon), the impact ionization (generating electron/holes) is large enough to damage the device. Such a field spike is therefore generally unwanted.
Generally therefore known devices may have low breakdown voltages and low operating frequencies or may suffer from the occurrence of field spikes and the associated problems.